Generally, the number of rotations of the wheel at the bicycle is detected mechanically by use of a tire or rim of the wheel. In other words, a drive unit is driven by the tire or rim of the wheel to transmit rotations thereof to a meter through a flexible wire, the meter indicating the number of rotations of the wheel.
Therefore, the drive unit detecting rotations of the wheel and the flexible wire connecting the drive unit and meter, are complex in construction and take much time to assemble. Also, a rotational resistance occurs between the drive unit and the wheel and requires an extra driving force for the bicycle's running, noises is and noises are generated.